The invention relates to a method in moistening of a web, in which liquid is applied in a predetermined point on the surface of a paper web or the like moving past the point, whereafter the web is guided to surface treatment. In particular, the invention relates to a method in which the surface of a paper web is wetted in a wetting unit in the paper machine before the web is calendered. The invention also relates to an apparatus for implementing the aforementioned method. Hereinbelow, the term paper web refers to all materials in the form of a flexible web made of fibrous material and capable of absorbing liquid.
After the paper has been dried, the surface structure of a web is made suitable by means of a mechanical treatment, calendering. There are several calendering methods, but it is common to all of them that the web is passed through one or several nips which are formed between two surfaces, typically between rotating roll surfaces. The purpose of the calendering is to improve the paper quality by pressing the paper into a fixed final thickness, and especially by smoothing its surface. As is well known, the mouldability of the fibres contained in the paper or paperboard, the xe2x80x9cplasticizationxe2x80x9d of the web in connection with the calendering, can be improved by increasing the temperature and/or moisture. The mouldability of the polymers contained in the paper can be increased by raising the treatment temperature to or above their glass transition temperature. By increasing the moisture content it is, in turn, possible to lower the glass transition temperature. It is often advantageous to restrict the impact of the temperature and moisture only on either surface layer or on both surface layers of the web, wherein the mouldability of their fibres can be improved without excessively affecting the central layer of the web. As a result of this, a known procedure is the wetting of the surfaces of the web before the calendering of the web. Another known procedure is the adding of steam before the nip between the calender rolls, wherein a preheating of the surface is also attained. Calendering processes are presented for example in the European patent 617165.
At present, the manufacture of SC paper requires so-called overdrying of the paper. This means that the paper is dried before calendering into a moisture which is smaller than its moisture of use. Maximally this means that the paper is dried very dry down into a moisture range of 1.5 to 2.5.
Overdrying is well founded, because the moisture expansion potential of the paper is reduced when the smallest moisture content which it experiences during the papermaking is reduced. Small moisture expansion potential improves the printability of the product. Furthermore, at present, overdrying and re-wetting produce better profiles in view of calendering when compared to drying of the paper directly to the target inlet moisture of the calendering.
Particularly in the manufacturing processes of SC paper the paper has to be wetted to attain the correct target moisture. In Off-line processes the wetting and the location in which it is conducted are not very crucial, because the moisture profiles, tensions and other corresponding variables of the paper have the time to be sufficiently equalized in the reel field before supercalendering. In on-line processes, in turn, it is important to reach the correct moisture of the paper before calendering.
The problem in the wetting of a web in the form of a continuous strip is that it is difficult to set the water content absolutely accurately to attain the desired moisture gradient in the thickness direction of the web. There is a risk that the moisture gradient is set too low, and the flattening effect of the calendering is unnecessarily directed to the central layer of the web instead of being directed in a most appropriate manner to the surface fibres of the web. However, in view of the final quality of the paper or paperboard, it is important that the calendering is successful. Thus, a correct dosage of water is the basic condition for a successful surface treatment.
To attain a sufficiently steep moisture gradient, it is possible to use arrangements known as such which relate to the structure and/or placement of wetting devices, but despite these arrangements the moisture gradient can be disadvantageous when the web enters the calender. Thus, the problems relate to the build-up of a correct moisture gradient as a result of the absorption of water taking place after the wetting devices. It is possible that after the wetting the absorption of water in the web before the calender is insufficient, or too much water is absorbed in the web. Similarly, it is a problem that with water the desired effect is not attained in the calendering. This may result in that the web remains unevenly wetted, the central layer of the web is unnecessarily wetted, or the fibres will be insufficiently moulded in the nip.
A known manner is to arrange the wetting devices in connection with the calender very close to the calender nip, as is presented e.g. in the European patent 617165 and in the U.S. Pat. No. 4,945,654.
It is a purpose of the invention to reduce the aforementioned problems and to introduce a method by means of which the intended wetting can be achieved. To attain this purpose, the method according to the invention is primarily characterized in that liquid is applied at such an early stage that the fibres exposed to wetting in the paper web or the like have time to absorb the liquid at least 80% of the total amount of liquid they are capable of absorbing before the surface is treated. The web is guided to surface treatment in the moisture content obtained by adding the liquid.
The period of time during which the wetting is effective after the wetting can be influenced especially by adjusting the distance travelled by the web from the wetting point, which can be implemented e.g. by changing the respective location of the rolls in the roll system guiding the web in such a way that the overall length of the path of the web in the roll system is changed.
The invention is based on the observation that in the end, only an optimal influence time of the wetting on the web when it travels from the wetting devices to the calender, is capable of ensuring the intended moisture effect. The web is wetted into the target moisture, and it is guided in this moisture into the calender nip.
Other features characteristic to the method according to the invention appear in the appended dependent claims, which present additional adjustment manners and possibilities to influence the uniformity of the wetting.
The apparatus according to the invention comprises a device for adding liquid, which is placed at such a distance from the device conducting surface treatment, taking into account the speed of the web, that the liquid added to the web will have time to be absorbed in the fibres exposed to wetting in the web at least 80% of the total amount of liquid which the fibres are capable of absorbing.